Valve has added subscriptions to Steam. The option is used in Darkfall Unholy Wars, a MMOG to paid 15euro a month. The system will be used for more games, however Valve had not given other examples.
It is not the first time that Valve through Steam sells a game where a subscription is handled. However, it is the first time that the company itself the option to settle payments through Steam. The game costs 40 euros and then another 15 euros per month to keep playing. The registration, renewal and cancellation of subscription proceeds through Steam and Valve put the working of the system in a faq.